A plastic polygon mirror manufacturing method in which a polygon mirror having a through hole provided at its center is molded by injecting molten plastic through a plurality of gates into a cavity of a mold is known in the art. Arrangement of the gates in the mold (i.e., of gate marks on the final castings) are typically designed such that when seen from a direction of an axis of rotation of a polygon mirror to be molded, each of the gates (or gate marks) is located on a straight line passing through the center of the polygon mirror and a center of each of reflecting surfaces of the polygon mirror. In this arrangement, the gates are located between the through hole and the centers of the reflecting surfaces of the polygon mirror, and thus a distance between each gate and a corresponding reflecting surface is short. Accordingly, the reflecting surfaces, generally desired to be approximated to an ideal shape specific to its design specifications, are liable to degrade in accuracy of its shape because injection of the plastic into narrow spaces sandwiched between the through-hole forming portion of the core and the reflecting-surface forming surfaces of the mold raises the fluid pressure of the plastic in the portions corresponding to the centers of the reflecting surfaces to be formed.